In the hippocampus, neural stem cells (green) sit in a layer below their progeny, the granule neurons (red). When activated by extrinsic stimuli, they enter mitosis and generate neuron progenitor cells, which eventually mature into neurons and migrate into the layer above. The number of neural stem cells in the hippocampus decreases over time, possibly contributing to the cognitive impairment associated with aging. One hypothesis is that, after a rapid series of divisions, these neural stem cells disappear via their conversion into astrocytes. Image: Section of a mouse hippocampus imaged with Zeiss LSM 50 confocal microscope with a 40X C-Apochromat water-immersion objective lens (N.A. value 1.2, working distance 220 microns) at 62x magnification. Brain slices were fixed in 4% paraformaldehyde, immunolabeled, and then cleared in FocusClear (CelExplorer, Taiwan).